The present invention relates to implantable medical devices, and more particularly to an implantable cardioverter defibrillator (ICD) configured to provide a low pain defibrillation waveform.
An ICD continues to be a relatively large device for implantation in the human body. The size of the ICD is primarily determined by the battery and capacitors used therein. The size of the battery (or batteries, in some instances) and capacitors, in turn, is determined by the shock energy requirements for a defibrillation pulse. Thus, a design approach that reduces the energy requirements for defibrillation results in a direct reduction in the overall ICD size.
In existing ICD devices, the defibrillation waveform or pulse used to deliver a defibrillation shock to the heart is generated by first charging the equivalent of a single capacitor (most ICDs use two capacitors connected in series to function as a single capacitor, thereby reducing the working voltage requirements for each capacitor of the series stack, as explained below) to a desired charge level (voltage) and then discharging the single capacitor through the cardiac tissue for a prescribed period of time during a first or positive phase of the defibrillation waveform, and then reversing the polarity of the discharge for a second prescribed period of time during a second or negative phase of the defibrillation waveform, thereby producing a biphasic stimulation pulse or waveform. It should be noted that in this context the term xe2x80x9csingle capacitorxe2x80x9d is used to refer to a single capacitance, which may be, and usually is obtained by a hardwired connection of two capacitors in series such that the two series capacitors always function and act as though they were a single capacitor. (Two or more capacitors are connected in series in this manner in order to achieve a higher working voltage for the series-connected capacitor. That is, when two capacitors are connected in series, and each has a working voltage of, e.g., 375 volts (V), then the overall or total working voltage of the series combination becomes 750 V.)
The purpose of applying a defibrillation shock to the heart is to shock the heart out of a state of fibrillation, or other non-functional state, into a functional state where it may operate efficiently as a pump to pump blood through the body. To this end, the positive phase of the biphasic waveform is preferably a very high voltage that serves to synchronously capture as many heart membrane cells as possible. See, Kroll, xe2x80x9cA minimum model of the signal capacitor biphasic waveformxe2x80x9d Pace, November 1994. The negative phase of the biphasic waveform, in contrast, simply serves to remove the residual electrical charge from the membrane cells and bring the collective membrane voltage back to its original position or value. See, e.g., Kroll, supra; Walcott, et al., xe2x80x9cChoosing The Optimal Monophasic and Biphasic Wave-Forms for Ventricular Defibrillationxe2x80x9d, Journal of Cardiovascular Electrophysiology (September 1995). A biphasic pulse generator of the type used in an ICD device is shown, e.g., in U.S. Pat. No. 4,850,357, issued to Bach, Jr.; and U.S. Pat. No. 5,083,562, issued to de Coriolis et al.
When a voltage shock is first applied to a membrane cell, the membrane does not respond to the shock immediately. Rather, the cell response lags behind the applied voltage. This time lag is more or less predictable in accordance with the Blair membrane model. See, e.g., Blair, xe2x80x9cOn the intensity-time relations for stimulation by electric currents Ixe2x80x9d, J. Gen Physiol., Vol.15, pp. 709-729 (1932), and Blair, xe2x80x9cOn the intensity time relations for stimulation by electric currents IIxe2x80x9d, J. Gen Physiol., Vol. 15, pp. 731-755 (1932); Pearce et al., xe2x80x9cMyocardial stimulation with ultrashort duration current pulses,xe2x80x9d PACE, Vol. 5, pp. 52-58 (1982). When the applied voltage comprises a biphasic pulse having a constant voltage level for the duration of the positive phase (a condition achievable only when the voltage originates from an ideal battery), the membrane cell response to the positive phase reaches a peak (i.e., is at an optimum level) at the trailing edge of the positive phase. Unfortunately, when the applied voltage originates from a charged capacitor, as is the case for an ICD device, the applied voltage waveform does not remain at a constant voltage level, but rather has a significant xe2x80x9ctiltxe2x80x9d or discharge slope associated therewith. Such tilt or slope causes the peak membrane cell response to occur at some point prior to the trailing edge of the positive phase, which is less than optimum. What is needed, therefore, is a way to optimize the applied voltage waveform so that a maximum membrane cell response occurs coincident with, or nearly coincident with, the trailing edge of the positive phase.
It is known in the art to switch the capacitors of an ICD from a parallel configuration during the positive phase of a biphasic defibrillation pulse to a series configuration during the negative phase of the biphasic defibrillation pulse. See, e.g., U.S. Pat. No. 5,199,429 (FIG. 7A) and U.S. Pat. No. 5,411,525. While such action produces a defibrillation waveform having a somewhat different shape, i.e., a waveform having a leading edge voltage of the second or negative phase which is approximately twice the trailing edge voltage of the first or positive phase, such action does little to achieve a maximum cell membrane response coincident with the trailing edge of the first or positive phase.
It is also known in the art to sequentially switch capacitors in an ICD device in order to allow waveform xe2x80x9ctailoringxe2x80x9d, e.g., prolong the positive phase duration by sequentially switching in a second charged capacitor as shown in FIG. 6A of U.S. Pat. No. 5,199,429, or by sequentially switching in second, third and fourth charged capacitors, as shown in FIG. 6C of U.S. Pat. No. 5,199,429. However, such xe2x80x9ctailoringxe2x80x9d still does not address the main concern of achieving a maximum cell membrane response coincident with the trailing edge of the positive phase.
It is thus evident that what is needed is a capacitor switching scheme and/or method for use within an ICD device which achieves a maximum cell membrane response near or coincident with the trailing edge of the positive phase.
It is also desirable to provide an ICD that is as small as possible. The limiting factor on ICD thickness is the diameter of the high-energy capacitors. As indicated above, current ICDs typically use two electrolytic capacitors. Current technology in electrolytic capacitors limits the stored voltage to about 450 V per capacitor. Therefore, the current approach is to use two large (200 xcexcF or more) capacitors to achieve the stored energy of 25J-40J required for defibrillation. Therefore, the thickness of the ICD is determined by the thickness of the large capacitors. There is thus a need for an ICD construction, which would permit the needed energy for defibrillation to be stored in the ICD, while allowing a thinner ICD thickness.
The inventions described in predecessor U.S. patent application Ser. No. 09/073,394 advantageously address the above and other needs. In particular, that patent application described a technique for generating a highly efficient biphasic defibrillation pulse by switching at least two charged capacitors from a parallel connection to various combinations of a parallel/series connection or a series connection during the first phase of the defibrillation pulse. A stepped-up voltage during the first phase, in turn, gives an extra boost to, and thereby forces additional charge (current) into, the cardiac tissue cells, and thereby transfers more charge to the membrane of the excitable cardiac cell than if the capacitors were continuously discharged in series. Phase reversal is timed with the cell membrane reaching its maximum value at the end of the first phase.
The inventions described in U.S. patent application Ser. No. 09/976,603 are directed to achieving still other advantages. More specifically, the inventions of that patent application were directed techniques for generating a defibrillation waveform that requires even less shock energy to reach the myocardial defibrillation threshold so that battery power can be saved and device longevity improved, while still providing effective defibrillation. Techniques were also described for generating a defibrillation waveform that reduces the total time required to reach the myocardial defibrillation threshold thereby permitting the patient to be defibrillated more quickly.
Although the techniques of the predecessor patent applications are quite effective in generating a wide variety of useful defibrillation waveforms, room for further improvement remains. A significant problem with conventional defibrillation techniques is the defibrillation pulses cause substantial pain to the patient. In many cases, the patient is unconscious by the time the shock is administered and hence the pain is experienced upon the patient regaining consciousness. In other cases, however, the shock is administered while the patient is still conscious. In either case, it would be highly desirable to reduce the pain experienced by the patient.
U.S. Pat. No. 5,906,633 entitled xe2x80x9cSystem for Delivering Rounded Low Pain Therapeutic Electrical Waveforms To The Heartxe2x80x9d by Mouchawar et al. provides various systems and techniques for reducing patient pain by eliminating sharp voltage peaks in the shocking pulse waveform. More specifically, the patent describes a system for delivering a low pain waveform that is biphasic and has rounded leading and trailing edges. The rounded leading and trailing edges are believed to decrease the discomfort experienced by the patient. In one embodiment, the circuit has two capacitors connected in parallel with one another and with an H-bridge. The two capacitors are connected via a switch that can be closed so as to simultaneously charge one capacitor from the other while simultaneously applying voltage to the H-bridge. The circuit also includes a dump resistor that can be connected in parallel with the capacitors so as to increase the rounding of the trailing edges of the waveform. In another embodiment, controllable switches can also be included so as to be able to connect the capacitors in series and apply a sharp peak defibrillation waveform to the heart. U.S. Pat. No. 5,906,633 is incorporated by reference herein in its entirety.
Although the system of U.S. Pat. No. 5,906,633 is effective is providing shocks that yield reduced pain, room for further improvement remains, particularly insofar as the generation of shocks for defibrillation is concerned. In particular, the circuit provided therein is fairly inefficient because much of the energy stored in the capacitors is not used in the shocking pulse and is instead lost as heat. Also it requires fairly large values for either voltage or power so as to achieve sufficient filtering to provide a smooth waveform shape. Alternatively, a larger capacitor can be used, but that requires larger ICD size and weight.
Accordingly, it would be desirable to provide alternative techniques for generating rounded shocking waveforms and it is to these ends that aspects of the invention of the present CIP application are directed. In particular it is desirable to exploit the stepped waveform techniques of the predecessor patent applications summarized above for generating low pain rounded waveforms and further aspects of the invention are directed to novel waveform shapes achieved thereby.
In accordance with a first aspect of the invention, systems and methods are provided for generating a rounded low-pain waveform formed of multiple steps or segments. In a system embodiment, a shocking circuit is provided that includes a set of capacitors, a resistive-capacitive (RC) filter, and low pain waveform control unit connected to the capacitors and operative to selectively discharge the capacitors through the RC filter to generate the rounded, multi-step defibrillation pulse waveform.
In one example, dual shocking capacitors are configured so as to be discharged either in parallel or in series during the positive phase of the pulse waveform. The low pain waveform control unit operates to first discharge the capacitors in parallel to generate a first step or portion of the positive phase of the waveform while periodically shunting a portion of charge through the RC filter to reduce the peak voltage of the first step. Then the low pain waveform control unit discharges the capacitors in series to generate a second step of the positive phase of the waveform while also periodically shunting a portion of charge through the RC filter to thereby also reduce the peak voltage of the second step. With this circuit arrangement, a rounded, multi-step waveform can readily be generated for use within an ICD without requiring a high voltages or large capacitors.
In accordance with a second aspect of the invention, systems and methods are provided for generating a shocking waveform that approximates a monotonically increasing input waveform shape. In an exemplary embodiment of the method, steps are performed so as to generate a waveform having a positive phase that approximates an input waveform shape having an initial portion increasing sharply from zero voltage to a initial voltage (Vinitial), a central portion increasing exponentially from the initial voltage to a peak voltage (Vpeak), and a tail portion decreasing sharply back to zero voltage. The central portion of the target waveform being approximated has an exponential shape represented by:
Vwaveform=Vinitial+(Vpeakxe2x88x92Vinitial)*(1xe2x88x92exe2x88x92t/T).
The monotonically-increasing waveform is believed to be particularly effective in reducing patient pain. The monotonically-increasing waveform can be generated using the dual capacitor system summarized above configured to produce a single monotonically-increasing waveform from the two steps of the multi-step waveform to thereby achieve significant pain reduction while also gaining the benefits of the use of multi-step waveforms.
In accordance with a third aspect of the invention, systems and methods are provided for generating shocking waveforms that approximate any input waveform shape, rounded or otherwise. In a method example, a shocking waveform is generated by inputting a waveform to be approximated then increasing a magnitude of a voltage of an output shocking waveform as a function of time. The magnitude of the voltage of the shocking waveform is compared to a magnitude of the voltage of the input waveform as a function of time and, whenever the magnitude of the shocking waveform exceeds the magnitude of the voltage of the input waveform, the magnitude of the shocking waveform is decreased until it again falls below the voltage of the magnitude of the input waveform. These steps are repeated so that the magnitude of the output shocking waveform generally approximates the input waveform.
In one example, the method is employed within a defibrillator having a shocking capacitor, a resistive-capacitive (RC) filter, and a chopping switch interconnecting the shocking capacitor and the RC filter. The magnitude of the voltage of the shocking waveform is decreased whenever the magnitude of the shocking waveform exceeds the magnitude of the voltage of the input waveform by opening and closing the chopping switch so as to produce an output from the RC filter that approximates the input waveform.
With this technique, virtually any desired positive-phase waveform shape can be approximated. Preferably, the technique is employed to generate the monotonically-increasing waveform summarized above to reduce patient pain. Also preferably, the technique is exploited using the dual capacitor multistep shocking system also summarized above to permit the use of relatively small capacitors using relatively low voltages.